In conventional technologies in the optical communications and optical information storage fields, mirrors having an inclination angle of 45.degree. were primarily used as a member for optical path altering use. In addition, developments in micro-optics and optical integration technologies in recent years have urged studies to fabricate a mirror for optical path altering use with an inclination angle of 45.degree. on optical components such as a hybrid integrated element and a monolithically integrated element. According to those studies, the mirror is provided on such optical components as a tapered surface having a 45.degree. inclination angle.
Various methods of tapering a surface are suggested, including the planar process using the same printing technology as that used in semiconductor fields. An example of the planar process is disclosed in OPTICAL PLANES AND REFLECTORS, ANISOTROPICALLY ETCHED IN SILICON (The 7th International Conference on Solid-State Sensors and Actuators) and other literatures.
Problems with the tapering of a surface by means of a planar process include poor surface precision, such as lack of smoothness on the tapered surface, and the incorporating method thereof into products. Specifically, the method disclosed in the aforementioned paper, despite its mass-producing capability of tapered surfaces by means of anisotropic etching of an Si substrate and seemingly easy integration into a semiconductor and other electronic components, falls short of providing satisfactory levels of surface precision and a complete matching of the manufacturing process with the semiconductor process.
Accordingly, in order to solve those problems, new methods have been developed recently to taper a surface by processing a high polymeric organic film fabricated in a monolithic manner on a substrate.
Examples of such methods include: a method of tapering a resist, which is disclosed in Japanese Laid-Open Patent Application No. 7-135142/1995 (Tokukaihei 7-135142, published on May 23, 1995); a method of tapering by dry-etching a lower layer by means of a tapered resist, which is disclosed in Japanese Laid-Open Patent Application No. 8-241889/1996 (Tokukaihei 8-241889, published on Sep. 17, 1996); a method of tapering by processing a polyimide film with a dry-etching technique, which is disclosed in J. Electrochem. Soc. Vol.130, No.1, pages 129 to 134; and a method of tapering by cutting a high polymeric organic film with a diamond cutter so that a mirror is fabricated with a reflective surface having an inclination angle of 45, which is disclosed in MOC/GRIN' 97 TOKYO, P2, pages 390 to 393.
However, the surface tapered according to any of the preceding methods still fails to provide satisfactory levels of smoothness or finishing conditions and suffers from side production of foreign bodies, such as residua and cut-out dust, and from damage incurred on elements.
Accordingly, to solve the above problems, the use of a laser is suggested as a method of tapering a surface.
Here, a brief explanation will be given about processing technology using a laser.
Typically, ultraviolet rays are used to excite organic high polymer and thereby cut internal bonds thereof, or establish bonding between separate organic molecules together. Especially, pulse lasers having wavelengths in the ultraviolet region of the spectrum, having high levels of energy, can decompose and evapourate the organic high polymer instantly by cutting carbon bonds of the organic high polymer. The high polymeric organic material can be optically excited at a surface thereof in high concentration, by irradiating the high polymeric organic material with a pulse laser, which produces high luminance light 10.sup.8 to 10.sup.9 times as bright as normal light such as a mercury lamp and a xenon lamp. Especially, if luminance exceeds a certain threshold value, a phenomenon called abrasion occurs where the high polymer compound is decomposed at their bonds explosively into decomposed segments that will change into a plasma state and fly in all directions at supersonic velocity while emitting light.
An suitable method of patterning a high polymeric organic film is an abrasion process using an excimer laser. An excimer laser is a pulse laser having a wavelength in the ultraviolet region of the spectrum and a pulse width of 10 ns to 20 ns, and can be used to process a high polymeric organic film with non-thermal techniques when modified so as to produce high luminance light having an energy density of a few hundred mJ/cm.sup.2 or greater.
Since there chiefly occurs an optical decomposition reaction in the excimer laser process as above, etching can be performed with little adverse thermal effect. Therefore, in comparison to RIE (Reactive Ion Etching) and other laser-associated processing techniques (YAG laser and carbon dioxide laser), excimer laser processing techniques are characterised by cleanness of processed shapes: i.e., the pattern in bodies to be processed (hereinafter will be called target bodies) does not melt, and few residua result from the process. In other words, unlike etching by a thermal process, that is, a technique to process material with heat, such as YAG laser processing and carbon dioxide laser processing techniques, excimer laser processing techniques are characterised by its ability to perform clean and precise processing.
A typical example of organic insulating films used for print substrates and optical waveguide elements is made of polyimide. In such a case, precise processing is facilitated if an excimer laser is used to fabricate a hole in the polyimide insulating film of a print substrate or to fabricate an optical waveguide from translucent polyimide.
Etching with an excimer laser is hence characterised as below:
1) The cross-section of the part that has been subjected to etching is smooth and clear-cut, and does cause incur thermal damage to the surroundings. PA0 2) The shape and position can be controlled highly freely in micron levels. PA0 3) The depth in etching can be controlled highly precisely up to about .+-.0.1 .mu.m. PA0 4) Radiation atmosphere can be created in any of the following states: in the air, in a reduced pressure state, and in a specific gas atmosphere.
An example of a tapering process using such an excimer laser is disclosed in Japanese Laid-Open Patent Application No. 4-330404/1992 (Tokukaihei 4-330404, published on Nov. 18, 1992) titled Method of Manufacturing Diffraction Grating (Conventional Technology A), and Japanese Laid-Open Patent Application No. 8-155667/1996 (Tokukaihei 8-155667, published on Jun. 18, 1996) titled Processing Device (Conventional Technology B).
Conventional Technology A discloses a method of fabricating a step of a triangular shape by irradiating a polyimide film with a laser that has passed through a mask from two oblique directions.
Conventional Technology B discloses a method of tapering a surface by working on the mask drive mechanism and the mask pattern of a laser processor so as to spatially altering the irradiated part of the target body, where the target body is irradiated with a laser beam, as well as the number of laser pulses emitted onto the target body.
According to Conventional Technologies A and B, a three-dimensional surface can be fabricated by highly precisely control the position, depth, and other values with respect to the processing of the target body.
Nevertheless, Conventional Technologies A and B have problems as follows.
According to Conventional Technology A, a tapering process is done by, for example, irradiating an organic material film 104b that is placed on a sample 104a, as a target body 104 on a stage 103, with a laser beam (light flux) emitted from an excimer laser exciting device 101 obliquely at a predetermined inclination angle by means of a movable mirror 102 as shown in FIG. 21.
In such a case, as shown in FIG. 22, a reaction product 106 sticks in the vicinity of a tapered surface of the target body 104. This is presumably because the processed part of the organic material film 104b is decomposed by laser abrasion into decomposed segments 105, which vapourise and ascend. Then, the decomposed segments 105, as they veer off the light flux, plunge at supersonic velocity due to cooling effect in the vicinity of the area where the processing is taking place.
The reaction product 106 clogs contact holes for wiring fabricated in the organic material film 104b for example, thereby reducing yields. Another problem possibly occurring from the reaction product 106 that is left over in the area where the processing is taking place is improper wire connections.
Moreover, if the reaction product 106 has a light absorbing property, the reaction product 106 sticking to optical components increases optical loss in the optical components, which is another problem.
Further, the reaction product 106, if once having stuck to the organic material film 104b, cannot be completely removed by washing and cleaning in an ordinary manner.
A tapering process according to Conventional Technology B entails the same problems as the tapering process in accordance with aforementioned Conventional Technology A, as well as the following problems.
According to Conventional Technology B, a V-shaped groove that will constitute the tapered shape can be formed by altering the shape of the mask pattern and moving the mask. Therefore, a tapering process requires the laser processing device to be equipped with a mask moving mechanism, which needs to be manufactured specifically for that purpose.